An induction heating coil normally comprises a core and a cable or a tube that is wound around the core. Typically, the core is of ferrite or laminated iron, and the material of the cable or the tube includes copper. It is possible to use the induction heating coil without the core, but particularly when the coil does not circumfere the component, the core improves the magnetic coupling to the component drastically.
In induction heating, an alternating current from an electric power source creates an alternating magnetic field. The alternating magnetic field induces eddy currents that heat a component. The magnetic field is arranged to alternate at a certain controllable frequency, and this frequency of the magnetic field determines the penetration depth of the induction heating into the component. The higher the frequency is, the lower is the penetration depth. To obtain the desired heating effect, the component must be an electric conductor. If the component includes ferromagnetic material, such as iron, cobalt, nickel, or their alloys, the penetration depth is reduced and the component acts as a higher resistance load. This helps to increase the efficiency of the induction coil.
The efficiency of the heating depends, among other things, on the electric losses in the coil. The use of litz cables as coil windings instead of hollow, water-cooled copper tubes has the advantage of low coil losses. A litz cable can be wound in several layers without producing excessive losses due to the fine threads of the litz cable that have a diameter smaller than the penetration depth of copper at the actual frequency. In contrast to a bobbin made of a hollow copper tube that has to be wound in one layer only to avoid excessive losses, a litz cable bobbin can be made with a large turn number and a larger effective copper area and thus reduced copper losses. However, to reach a desired power in a litz cable coil, the losses in the winding and core is normally at a level that requires forced cooling to protect the insulation material of the core, or the cable forming a bobbin, from over-heating.
Coils comprising litz cables are discussed e.g. in U.S. Pat. No. 5,101,086 and U.S. Pat. No. 5,461,215. U.S. Pat. No. 5,101,086 deals with coils in a frequency range from 12 to 25 kHz and discloses an electromagnetic inductor with a ferrite core for heating electrically conducting material. The electromagnetic inductor includes a litz cable and a water cooled magnetic flux concentrator tube which is arranged to circulate water and cool indirectly a coil wound of the litz cable. The cooling concentrator tube is disposed around the coil and insulated from it with a synthetic resin. The electromagnetic inductor is utilised for instance on calenders.
The inductor with concentrators acts as a combined transformer and coil. The litz cable winding is the primary winding. The concentrator is a combined one-turn secondary and a one-turn coil. The inductor can have a high input impedance because of the high turn number possible in the primary litz cable winding, but the losses will be on the level of a coil made of a hollow copper tube and thus several times as high as in a pure litz cable coil. This will have a significant influence on the efficiency of the coil, even if the component is of ferromagnetic material.
The publication U.S. Pat. No. 5,461,215 discloses a litz cable surrounded by a coolant tube. A fluid for removing heat generated by the litz cable is conveyed through an annular space between the litz cable and the coolant tube. The coolant is thus in direct contact with the litz cable. This publication deals with high current litz cables whose diameters vary from 9.5 mm to 14 mm. The current varies from 700 to 1000 A, and the frequency is 300 kHz.
The induction heating coils comprising litz cables which include a cooling system are rather complicated. In addition, the coolant tubes having cooling medium flowing inside them and around the litz cables within the coolant tubes have large diameters. The large diameter causes restrictions on the number of windings and thus the power is not achieved when the current is limited. If the diameter of the cable is reduced, the amount of copper in each thin, electrically insulated thread is reduced and thus energy losses increase. Due to the greater losses, more heat is generated and thus more cooling is required. Better cooling necessitates greater fluid volume, or the current density must be limited to a level at which the prevailing cooling is adequate. Furthermore, the cables with large diameters have restrictions on bending, e.g. they cannot be wound around a core with a small diameter.